The present invention relates generally to electromagnet switching devices and, more particularly, to a single coil solenoid having a permanent magnet with bi-directional assist.
Electromagnet switching devices such as solenoids are commonly used in a number of applications such as shut-off devices for fuel or other types of fluid pumps. Solenoids are frequently used in engine applications in the throttle, choke, valve, clutch, and overspeed protection assemblies. As such, solenoids are typically found in engine driven products such as boats, lawn equipment, automobiles, generators, and the like.
Solenoids are designed to convert electrical energy into mechanical work. Typically, a movable armature or plunger reciprocates linearly from a first to a second position when current is induced in coil(s) in which the armature sits. The current induced in the coil(s) creates a magnetic field about the armature that induces movement in the actuator along one direction. In this regard, the armature may be connected to a device or piece of equipment such that when current is induced in the coil(s), the armature is caused to turn ON, turn OFF, open, or close the device.
Typically, solenoids include either a single coil of copper wire or a pair of coils of copper wire. In a single coil solenoid, when electric current is introduced, a magnetic field forms which causes movement of a plunger or armature. Typically, the magnetic field draws the plunger inward to a retracted or energized position. In a single coil solenoid, the current induced to create the magnetic field to cause movement of the armature or plunger must not only be sufficient to pull or push the plunger but also be sufficient to maintain the plunger in the energized position. A drawback of a single coil solenoid, however, is that when the coil is energized for long periods of time, the coil may overheat thereby rendering the solenoid inoperable. To overcome this drawback, dual coil solenoids are typically used for applications in which the plunger or armature may need to be maintained in an energized position for long periods of time.
A typical dual coil solenoid is shown in FIG. 1. Solenoid 10 includes a first or pull coil 12 and a second or hold coil 14. Generally, the first wound coil operates at a high current level to provide a maximum pull or push on plunger 16. The second wound coil is used to simply hold the plunger in place after the plunger has completed its stroke and requires less energy. The coils 12, 14 are typically fabricated from copper wire and the plunger is magnetic material with a coating or plating to resist wear, friction and corrosion. The amount of current required to maintain the plunger in a hold position is typically less than that needed to push or pull the plunger and, as such, a dual coil solenoid may be energized continuously without overheating. The coils 12, 14 as well as plunger 16 are typically positioned within a steel housing 18 that may include mounting brackets 20 for mounting the solenoid to a frame or other piece of equipment. Some solenoids further include a return spring 22 that is used to bias the plunger 16 in a de-energized position. As such, the magnetic force placed on the plunger through high current in coil 12 must be sufficient to overcome the bias of spring 22. For those solenoids incorporating a return spring 22, a flexible dust boot 24 is commonly used to enclose return spring 22 and is mounted or connected to the housing 18. At an opposite end of housing 18 is typically a double break switch 26 that is controlled to regulate which coil is energized. As such, switch 26 may be actuated such that dynamic control of current inducement in either the pull or push coil 12 or hold coil 14 is maintained. The double break switch 26 is typically sealed against dirt and moisture, and a housing or cover 28 secured to housing 18. Extending through cover 28 is a number of terminals 30 for connecting electrical leads to the solenoid.
As illustrated in FIG. 1, a typical solenoid is constructed with copper wire on a non-conductive, non-magnetic bobbin that creates a coil assembly. The coil assembly is assembled into a magnetically conductive shell that becomes an electromagnet when energized that may create a force on a movable magnetic object such as a plunger or armature. The force exerted on the plunger is directly proportional to the electrical current and the number of turns of wire on the bobbin. That is, the higher the number of ampere-turns, the greater the force imparted. From this proportional relationship, increasing the number of turns or increasing the current may increase the amount of force imparted. Some solenoids, which are particularly used in space constrained applications, utilize two separate coils on the same bobbin. As discussed above, these coils are typically referred to as a “pull” coil and a “hold” coil.
The pull coil, as described above, is designed to carry a very high current generate relatively high forces on the plunger or armature initially. Typically, this high amount of force is for a short period of time at which point the current is switched off to prevent the coil from overheating. The hold coil usually operates with a much lower current and takes advantage of the fact that the plunger requires much less energy to maintain the “pull” force exerted on the plunger or armature. Typically, the pull coils are switched off in different ways but two of the most common ways are either mechanically or electronically. That is, the mechanical switching method usually implements the plunger to interrupt the circuit at or near a zero stroke by opening a set of switch contacts that is a part of the solenoid. The placement of these contacts is critical as is their ability to handle high currents. Switch design has its own unique requirements that must be considered in the overall solenoid design further complicating the solenoid as well as adding cost and potential reliability concerns. On the other hand, electronically controlled solenoids may use relays or solid state switching devices to accommodate switching functionality. These electronic components, however, add costs to the solenoid. Another switching method that uses electronics implements a single coil of wound wire which is similar to a pull coil in that it uses high current to create a high initial force. The electronics therefore supply full power to the coil initially. When the plunger has reached full stroke, typically after a specified time period, the electronics start switching the current on and off at a relatively high frequency to reduce the effective current. This process is typically referred to as pulse width modulation and makes the single high current pull coil effectively also the low current hold coil. However, electronics not only add to the manufacturing cost of the solenoid but also increase the complexity of the solenoid.
It would therefore be desirable to design a solenoid having a single coil of wire that achieves both push/pull and hold functions without the additional cost and complexity of mechanical or electronic switch assemblies.